In ocean industries and in the offshore petroleum industry, in particular, it is necessary to locate marine vessels and structures in particular relation to the sea bed and maintain these structures in stabilized condition while operations are being conducted. In the case of well drilling for petroleum exploration and production, drilling rigs are typically located on floating vessels, semi-submersible vessels or bottom supported platforms. In each case, and especially so in the case of floating vessels and semi-submersible vessels, it is necessary to maintain the drilling rig in a rather precise location relative to the ocean floor, to thus maintain the rotating drill stem as straight as possible as it is being rotated by the drilling rig. Bottom supported platforms are widely used in relatively shallow water conditions, for example in water depths up to 600 feet. The lower portion of such bottom supported platforms is secured to the ocean floor by means of piles that are driven to desired depths. When wells are drilled in water depths exceeding 1000 feet, it is typical to employ floating or semi-submersible drilling vessels which are anchored in place by means of cables extending from various connection points on the vessel to anchoring devices that are secured to the ocean floor. These anchoring or mooring cables are typically connected to cable winches located on the drilling vessel thereby permitting cable adjustment sufficient to maintain the vessel positioned substantially over the well bore being drilled.
Wave action, wind and current acting upon floating vessels, and tall and slim marine structures cause lateral shifting or excursion of the marine structures. Because of the catenary that forms in conventional mooring cables, the forces of wind, current and wave action can cause tightening of the cables on one side of the vessel and consequent loosening of the opposite cables, thus allowing the vessel or structure to shift laterally. The amount of lateral shifting or excursion that occurs depends on the forces applied to the vessel and to the curvature present in the mooring cables or chains. Drilling activities can take place only when the structure or vessel is maintained within prescribed limits of lateral excursion. For example, a conventionally moored floating or semi-submersible vessel in 600 feet to 1,000 feet of water will be capable of lateral excursion in the order of 45 feet because of the catenaries defined in the mooring cables. Drilling activities can be conducted, however, only when drilling vessel misalignment above the well bore is maintained to within about three percent of water depth. Under circumstances where wind, wave action and current cause the vessel to shift laterally beyond the maximum prescribed for drilling, drilling operations must cease. When drilling operations are being conducted in marine environments where stormy conditions occur frequently, for example in the North Atlantic and North Sea areas, drilling rigs are frequently required to shut down simply because the weather conditions, wave action and currents cause lateral excursion of the drilling rig beyond acceptable limits. This, of course, is detrimental to the cost of drilling operations because the fixed costs of maintaining the vessel, equipment and personnel continues during such periods of inactivity. It is, of course, desirable to provide a mooring system for floating and semi-submersible drilling rigs which will significantly reduce the amount of down time that is presently due to adverse weather conditions.
It is desirable to provide a mooring system having no catenaries and thus efficiently minimizing lateral excursion in response to forces generated by wave action, wind and current and maintaining the structure stable within allowable limits.
As exploration for petroluem continues in the ocean environment, the need for drilling operations in water of greater depth becomes more desirable. At the present time, offshore drilling operations are being conducted from at least one bottom supported platform in water as deep as slightly under 1000 feet which is presently considered record water depth for bottom supported platforms. Actually, approximately 600 feet of water is considered to be the practical limit for bottom supported platforms from the standpoint of cost and productivity.
A tension leg platform system is being developed at the present time by Conoco, Inc., which is scheduled to be operating in the Hutton Field in the North Sea by 1984. The tension leg design concept utilizes a semi-submersible marine vessel and a plurality of structural members that link the platform to the sea floor in tension rather than compression. The tension leg system provides restoring forces that tend to recenter the vessel above the well being drilled if lateral excursion should take place. The tension legs secure the vessel in such manner that it is relatively insensitive to wave action from the standpoint of rising and falling. However, the vessel will be subject to lateral shifting in response to wave action, current and wind. For example, on the basis of model basin tests, the maximum excursion in a design storm with 98 foot, 17 second waves, accompanied by a 95 knot wind and 2.5 knot currents, all impinging on the platform from a 45.degree. angle, will be 79 feet. The tension leg platform system is discussed in the February, 1980 issue of Ocean Industry, at pages 35-39. Although a platform of tension leg design will have less lateral excursion than conventionally moored floating and semi-submersible vessels, better productivity of tension leg drilling systems could be accomplished if lateral excursion were further restricted by a catenary free mooring system.
The catenary free mooring system of this invention is protected against the otherwise damaging effects of sagging and snap loads by means of automatic sagging preventers which interconnect each of the moors to the vessel or structure being moored. The automatic sagging preventer applies a pretension load to each of the moors while at the same time accommodating differences in the length of the individual moors. Moreover, the automatic sagging preventer maintains each of the moors under tension even during lateral excursion of the vessel or structure such as is induced by environmental conditions such as wind loads, wave action and current.